linux-kernel - Re: [PATCH v3 2/3] xen-blkfront: add a parameter for disabling of persistent grants

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Date:   Tue, 22 Sep 2020 16:31:17 +0200
From:   SeongJae Park <sjpark@...zon.com>
To:     Jürgen Groß <jgross@...e.com>
CC:     SeongJae Park <sjpark@...zon.com>, <konrad.wilk@...cle.com>,
        <roger.pau@...rix.com>, SeongJae Park <sjpark@...zon.de>,
        <axboe@...nel.dk>, <aliguori@...zon.com>, <amit@...nel.org>,
        <mheyne@...zon.de>, <pdurrant@...zon.co.uk>,
        <linux-block@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/3] xen-blkfront: add a parameter for disabling of persistent grants

On Tue, 22 Sep 2020 16:27:03 +0200 "Jürgen Groß" <jgross@...e.com> wrote:

> On 22.09.20 16:15, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@...zon.de>
> > 
> > Persistent grants feature provides high scalability.  On some small
> > systems, however, it could incur data copy overheads[1] and thus it is
> > required to be disabled.  It can be disabled from blkback side using a
> > module parameter, 'feature_persistent'.  But, it is impossible from
> > blkfront side.  For the reason, this commit adds a blkfront module
> > parameter for disabling of the feature.
> > 
> > [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability
> > 
> > Signed-off-by: SeongJae Park <sjpark@...zon.de>
> > ---
> >   .../ABI/testing/sysfs-driver-xen-blkfront     |  9 +++++++++
> >   drivers/block/xen-blkfront.c                  | 20 ++++++++++++++-----
> >   2 files changed, 24 insertions(+), 5 deletions(-)
> > 
> > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkfront b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> > index c0a6cb7eb314..9c31334cb2e6 100644
> > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkfront
> > @@ -8,3 +8,12 @@ Description:
> >                   is 32 - higher value means more potential throughput but more
> >                   memory usage. The backend picks the minimum of the frontend
> >                   and its default backend value.
> > +
> > +What:           /sys/module/xen_blkfront/parameters/feature_persistent
> > +Date:           September 2020
> > +KernelVersion:  5.10
> > +Contact:        SeongJae Park <sjpark@...zon.de>
> > +Description:
> > +                Whether to enable the persistent grants feature or not.  Note
> > +                that this option only takes effect on newly created frontends.
> > +                The default is Y (enable).
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index 91de2e0755ae..b2d3b1262404 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -1866,8 +1866,8 @@ static int talk_to_blkback(struct xenbus_device *dev,
> >   		message = "writing protocol";
> >   		goto abort_transaction;
> >   	}
> > -	err = xenbus_printf(xbt, dev->nodename,
> > -			    "feature-persistent", "%u", 1);
> > +	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
> > +			info->feature_persistent);
> >   	if (err)
> >   		dev_warn(&dev->dev,
> >   			 "writing persistent grants feature to xenbus");
> > @@ -1941,6 +1941,13 @@ static int negotiate_mq(struct blkfront_info *info)
> >   	}
> >   	return 0;
> >   }
> > +
> > +/* Enable the persistent grants feature. */
> > +static bool feature_persistent = true;
> > +module_param(feature_persistent, bool, 0644);
> > +MODULE_PARM_DESC(feature_persistent,
> > +		"Enables the persistent grants feature");
> > +
> >   /**
> >    * Entry point to this code when a new device is created.  Allocate the basic
> >    * structures and the ring buffer for communication with the backend, and
> > @@ -2007,6 +2014,8 @@ static int blkfront_probe(struct xenbus_device *dev,
> >   	info->vdevice = vdevice;
> >   	info->connected = BLKIF_STATE_DISCONNECTED;
> >   
> > +	info->feature_persistent = feature_persistent ? 1 : 0;
> 
> Just use the value.

Yes, I will.

> 
> With that changed:
> 
> Reviewed-by: Juergen Gross <jgross@...e.com>

Appreciate :)


Thanks,
SeongJae Park